Magnetic resonance imaging (MRI), nuclear magnetic resonance (NMR), and other magnetic resonance (MR) systems continue to become more sophisticated, powerful, precise, and complicated. MR systems rely on transmit coils to expose volumes to precisely controlled radio frequency (RF) energy. Like the larger overall MR system, RF transmit coils also continue to become more sophisticated, powerful, and complicated.
RF transmit systems continue to add more independent elements to correct B1 inhomogeneity, to reduce the specific absorption rate (SAR) at high field strengths (e.g., 3 T, 7 T), or for other reasons. However, adding more independent elements can be complicated by factors including inter-element coupling, cabling restrictions, and other factors. Element and coil design is further complicated by the unique and intense environment (e.g., 7 T magnetic field, rapid gradient switching, complex RF pulse sequences) in which RF transmit coils operate.
MR involves the transmission of RF energy. RF energy used to produce MR may be transmitted by a coil. Resulting MR signals may also be received by a coil. In early MRI, RF energy may have been transmitted from a single coil and resulting MR signals received by a single coil. Later, multiple receivers may have been used in parallel acquisition techniques. Using multiple receivers facilitates speeding up signal reception, which in turn may reduce scan time. Similarly, multiple transmitters may be used in parallel transmission techniques. Using multiple transmitters may facilitate speeding up a transmission process, which in turn may facilitate volumetric excitation, selective isolation, and other very high speed features. However, conventional parallel transmission techniques have encountered issues with scaling, fidelity, and synchronization.
Conventional systems may have relied on multiple, individually powered, single channel, analog-in-analog-out RF transmitters for parallel transmission. These systems tended not to scale well due to cabling duplication, power transmitter duplication, control duplication, and other issues. Even when a small number (e.g., 4) of transmitters were employed, these systems may not have produced desired fidelity. Additionally, conventional systems typically had poor isolation between coils, resulting in degraded performance.
Conventional systems may also have been limited by their use of relatively low power (e.g., <50 W), low efficiency class A or class AB amplifiers. While some systems may have included on-coil series and/or shunt-fed class-D amplifiers, even these conventional systems suffered from several limitations including inadequate detuning and low efficiency. Due, at least in part, to these limitations, conventional systems may not have produced desired levels of amplitude and/or phase control and thus may have had less than desirable fidelity.
Some RF transmit coils were therefore designed using a transmit amplifier on the transmit coil. Some RF transmit coils may even have used high efficiency amplifier topologies. While these approaches have led to improvements, designing circuits that include amplifiers is still constrained by factors including amplifier size, heat dissipation, and other factors.
The field of coil design using on-coil switched-mode amplifiers for parallel transmission is relatively new. For example, U.S. Pat. No. 7,671,595, which issued on Mar. 2, 2010 to one of these same inventors, presented an early on-coil switched-mode amplifier. U.S. Pat. No. 7,671,595 (“the '595 patent) is entitled “On-coil Switched-mode Amplifier for Parallel Transmission in MRI” and describes an on-coil current-mode class-D (CMCD) amplifier that may be used to produce MRI transmission-coil excitations at desired RF frequencies. The on-coil CMCD amplifier is capable of performing within or proximate to the bore of the MRI magnet or within less than one wavelength of the RF signal produced by the transmit coil or at other positions or locations. Providing an on-coil amplifier allows digital control signals to be sent to the coil assembly, improving synchronization between the transmission-coils while reducing interference, cross talk, physical space requirements associated with cables, and heating normally associated with parallel transmission MRI systems. The on-coil CMCD amplifier described in the '595 patent may be driven by signals produced by one or more linear pre-amplifiers.
Once this type of coil had been built, additional research using the new on-coil switched-mode amplifier revealed the need for further refinements including those presented in U.S. Pat. No. 8,294,464, which issued on Oct. 23, 2012 to one of these same inventors and in U.S. Pat. No. 8,294,465, which also issued on Oct. 23, 2012 to one of these same inventors. These patents describe improvements to the on-coil switched-mode amplifiers including the use of CMCD amplifiers, switched-mode pre-amplification, and amplitude modulation (AM) feedback for the on-coil switched-mode amplifiers. Once again, as these on-coil switched-mode amplifiers were built and used, further research revealed the need for further optimizations.